Try going to the walther forum on rimfirecentral.com
That is where I learned how to fix mine.
Basically, you take the two safety levers off and use a small file to remove some of the material from the flats (not the two prongs that stick up) so that the screws can be tightened down more.
By the way, I think you mean that "L" is your serial number prefix. The date code is on the barrel and will be two letters. Mine is "AF".